IBM5160 PSU output voltage issue

[Malc]

A few points that may help:
1) my PSU is like this one inside: https://www.vogons.org/viewtopic.php?t=73543

I have a schrack PSU that looks like that, 130W 220VAC, EG071068-0, P/N 1501438, Date 1985
I see NO Factory Re-work stickers on the case or inside, Of the 7 electrolytic caps, 6 are blue and one on the secondary side is black, That may have been replaced in the past ?.

On the bench powered up with no external load attached i measure the output voltages:
+5v = +5.40v
-5v = -4.96v
+12v = +11.36v
-12v = -11.72v
PG = +5.40v

I attach a load to the +5v line using a automobile bulb, And attach 2 hard drives, An ST-251 and a more recent Maxtor and measure the output voltages again.
+5v = +5.14v
-5v = -4.99v
+12v = +11.98v
-12v = -12.36v
PG = +5.14v

I would still try a different PSU if you have one.

 

[Ruud]

That looks very good!

 

[hozone]

I've moved this request to the eevblog forum, cause it's related more to electrical than functional part. Anyway... I've been able to make it works. Now I get almost 12.3V on 12V line and almost 5.2V on 5V line under working condition.
It happens to be a thermal controlled diode on the SMD additional board of the PSU. Having no schematic it was pretty difficult to find it out, but thanks to you and the people on eevblog now this PC is working again. If anyone is interested you can find the full post here: https://www.eevblog.com/forum/repair/vintage-130w-ibm5160-schrack-elektronik-eg07168-e-psu-repair/
Again, thank you.

 

[Hugo Holden]

Generally, it is ok to use a standard 6.2V zener as a voltage reference in a power supply. If you look at the temperature coefficients of zeners above that voltage they are positive, meaning the zener voltage all else equal, increases with heating. Below 6.2V it is generally a negative tempco.

So 6.2V is the magic number for a near zero tempco zener. Some 6.2V types though are especially selected and labelled for a close to zero tempco and sold as specific voltage reference diodes. There is quite a spread though in the 1N4735A part, you will find that if you tried a few, you would find one that landed the supply output voltages nearly bang on.